Limiter circuit



Sept. 26, 1950 A. THOMPSON LIMITER cmcurr Original Filed April 22, 1944Patented Sept. 26, 1950 LIMITER CIRCUIT Louis A. Thompson, Rocky River,Ohio, assignor to NEA Service, Inc., Cleveland, Ohio, a corporation ofDelaware Original application April 22, 1944, Serial No.

1949, Serial No. 100,490

4 Claims.

This invention relates to improvements in electrical circuits and moreparticularly to methods and apparatus that are especially useful in thetransmission of intelligence by wire or radio. This application is adivision of application Ser. No. 532,252, filed April 22, 1944, nowPatent No. 2,488,517, November 15, 1949.

It has become common practice to transmit pictures electrically by meansof picture transmitters which are connected to each other by telephonelines or the like. A machine that is designed to transmit picturesefiiciently over a telephone line may not easily be adapted to transmitpictures by radio and one adapted for radio transmission may not easilybe adapted to transmit over a telephone-line or the like.

It is well known that pictures in order to be reproducible with fidelitywhen transmitted by wire rely to a considerable extent on the perfectionof the line connection between the transmitter and the receiver. Thatis, during transmission, variation in signal, line noises and otherdisturbances are usually apparent in the final picture, and in someinstances have such effect thereon as to make the picture illegible. Itis particularly desirable in picture transmission for newspaperreproduction that as good a picture as possible be obtained becauseother losses occur during the reproduction, which, if coupled with thelosses taken during transmission, may

make the picture so poor that it cannot be used.

By my present invention I have provided an improved method and apparatusfor the transmission and reception of pictures either over the wire orradio whereby the picture is substantially unafiected. by line noises,changes in volume and in the case of radio transmission, by static,fading and the like.

In carrying out the invention I contemplate using a standard picturetransmitter and a standard picture receiver, and the apparatus about tobe described herein comprises an inverter for use during reception.However, it should be borne in mind that the certain parts of theinverter could be built into the receiver to provide a unitary device.Therefore, the invention in its broader aspects contemplates a newmethod of picture reception; and in its more specific aspects, a meansfor convertin existing machines so that the new method of reception canbe effected thereby.

A brief description of the function of the apparatus will assist in anunderstanding of the apparatus itself hereinafter describedspecifically.

To begin with, the picture transmitter may be Divided and thisapplication June 21,

of the character described in my Patent No. 2,284,027 of May 26, 1942.As described in the above patent, it contemplates scanning a picture toprovide a Signal in the output of the machine which may be an 1800 cyclesignal that varies in strength or amplitude in accordance with thereflection of light from the picture. This may also be considered forthe purpose of explanation as an 1800 cycle amplitude modulated signal,the amplitude modulations of which carry the intelligence. Ordinarily,this signal is sent from the picture transmitter to the picture receiverover the telephone lines or by means of land line connections. In thereceiver the signal is used to reproduce the picture. During the processof transmission, an amplitude modulated signal sometimes accumulatesundesirable components due to line noises or increases and decreases insignal strength, all of which affects the desired modulation componentand which results in defects in the finished picture.

Therefore,.instead of sending the signal out on the lines in the form ofan amplitude modulated wave, as it comes from the transmitter, I pass itthrough a converter as described in my aforementioned co-pendingapplication. In the converter I amplify the signal to a predeterminedlevel. Next the signal is demodulated. Then I utilize the demodulatedsignal to control a variable frequency oscillator. The variablefrequency oscillator and a fixed frequency oscillator are each coupledto a mixer demodulator which detects the difierence between the twooscillators and wherein the signal is again demodulated to provide afrequency modulated audio signal. This signal is then amplified andappears in the output of the converter. All of the foregoing is effectedin the converter system and the output may now be used to modulate anykind of a transmitter or may be sent by wire if desired without anychange. In the case a transmitter is modulated, it may be either aso-called amplitude modulated type transmitter, or it may be a frequencymodulated type of transmitter.

The signal after being transmitted by radio is received by any suitabletype of receiver, depending on the final type of transmitter used.During the transmission it may be efiected in many ways. It mayaccumulate static, noise and/or fading, all of which under ordinarycircumstances might be sufiicient to render the ordinary amplitudemodulated signal substantially useless for high fidelity reception ofpicture.

After being received, it is amplified together with the above mentionedundesirable defects and then passes through the inverter. In theinverter it passes through a cascade limiter which very greatly limitsthe signal, taking off all amplitude modulations. Next, it isreconverted to an amplitude frequency modulated hybrid signal, thenrectified and passed through a filter. Then this signal is combined withan 1800 cycle carrier. an 1800 cycle constant frequency, amplitudemodulated signal which may be supplied to the standard picture receiverwith a result that the picture is substantially the same as it wouldhave been had the facsimile transmitter been directly connected to thefacsimile receiver. Thus the signal is handled in such a manner that theusual losses which occur in transmission do not affect it.

It is therefore among the objects of my invention to provide anapparatus whereby a picture signal, such as is received by radiotransmission, may be inverted to enable a conventional picture receiverto reproduce the same.

I Another object of my invention is to provide an improved method ofreceiving pictures or intelligence whichis substantially unaffected bystatic, fading or the like.

Another object of my invention is the provision of an improvedfrequencymodulation system and method of operation thereof.

Another object of my invention constitutes the provision of an improvedmethod and apparatus for inverting a signal.

Otherobjects of the invention are the improvements of circuits embodiedin the invention and includes an improved limiter circuit, an improveddemodulation circuit and an improved signal reconstruction circuit;

Still other objects of the invention and the invention itself willbecome more apparent from the following description of a specificembodiment of the apparatus used to receive a signal which apparatusisillustrated by the accompanying drawings and 'forms a part of thisspecification. In the drawings:

Fig. 1 is a schematic drawing of a complete receiving system forreceiving and reproducing the signals from the apparatus of my copendingapplication; and

Fig.- 2 is a simplified view of a portion of the circuit of Fig. 1. 1

'Referring now to the drawings throughout which like parts aredesignated by like reference characters. The receiving end of the systemis best shown in Fig. l.- Here the signals, if the transmission is byradio, are received over a suitable receiver 50. Connected tothereceiver 50 is an amplifier 5! in which the signal is raised to a highvalue. At the output to the receiver the signal may contain certainundesirable components such as noise and static that may have beenpicked up during transmission, and in addition, the signal may besubject to severe fading. In other respects, however, the signal may bethe same as it was when it was used to modu late the transmitter; thatis, a frequency modulated signal having a swing between 1000 and 2500cycles.

Next, the signal is put through what I prefer to term a push-pullcascade limiter circuit. This is effected by feeding thesignal into theprimary of a push-pull transformer 52, the secondary of which has eachopposite ends connected through The signal has now become theresistances 53 and 53' to the grids 54 and 54 of the tubes 55 and 55.

The signal is built up to a high level to enable the limiter to functioneven in the presence of deep fading of the signal.

Each limiter tube may include a twin triode and the connections are suchthat each triode section is connected in cascade. The operation may bestbe understood by reference to the diagram in Fig. 2 which shows one-halfof the pushpull limiter circuit with the twin triode sections separated.Assuming that a fairly strong signal is being received, this signal isamplified to a high level and is applied to the grid 54 of the firsttriode section of the tube 55 through a series resistance 53. The grid54 is driven positive on the positive half of the signal which causesgrid current to flow in the grid-cathode circuit. This grid currentcauses an IR. drop across the resistor 53 with the result that thesignal into the tube is limited.

The anode voltage for the first triode section is supplied by the mannerin which the second triode section is connected. The anode 51' of thesecond triode is connected to the positive plate voltage supply, andnormally this tube draws current from the plate circuit to the cathode59 which is connected through a resistor 58 to the grid of the secondsection and the anode 51 of the first triode section. The circuit thenfor the cathode of the second section is through the first triodesection to the cathode 56 and thence to ground. The voltage drop acrossresistor 58 provides bias for the grid 60. Since the cathode 55 0f thefirst section is at ground potential,

the plate voltage at 51 is above ground potential due to the currentfiow in 58.

When the positive signal appears on the grid 54 the plate 5'! drawscurrent which is through the resistor 58, thus increasing the negativebias on the grid 60, which action decreases the normal plate current inthe second section thus reducing the plate voltage at 5T.

This still further increases the limiting action of the circuit. Thislimiting action is relatively large, depending; of course, on theamplitude of the signal. The final result is that the resultant waveform is devoid of all amplitude modulations caused by static, noise orother amplitude picked up previously in the system and is substantiallya square wave. Of course, on the other half of the wave the otherlimiter tube 55' is operating in a like manner.

The maximum limiting of the signal voltage may be as much as from 2000to 2*. In actual practice, I have found that a desirable method ofoperation is to raise the signal to a. relatively high value before itis limited, thus providing sufiicient signal to always provide limitingaction. In practice, I have found it desirable for the limiter to ,startfunctioning on voltages as low as 2 volts.

As a result of the limiting action, the output from the limiter issubstantially a constant voltage of varying frequency.

The output from the two limiter circuits are connected in push-pull tothe primary of the transformer 6 l the secondary of which feeds into asloping filter 62. This filter, as the name implies, has a 45 degreesloping characteristic. A signal having a frequency of 1000 cycles,which may bepicture white, will pass through the filter withoutany-substantial loss. Asthe frequency of the signal increases the lossbecomes greater, hence a 2500 cycle signal suffers a very substantialloss. Likewise, the signals between 1000 and 2500 cycles suffer a losssubstantially in direct proportion to their frequencies. The result thatthe signal in the output of the sloping filter 62 comprises a signalthat varies in amplitude in a manner corresponding to the originalamplitude modulation component. The signal at this point comprises ahybrid signal which consists of frequency and amplitude modulations.

The signal is then fed through the transformer 63, the secondary ofwhich isconnected to the amplifier tube 64 which may be another twintriode with the two sections connected in parallel, and in which thesignal is amplified.

The output from the tube 64 is connected to the transformer 65, thesecondary of which is connected to the anodes of the demodulator tube6?. This tube rectifies both halves of the signal which then appears onthe cathodes of thetube which are connected together, and is a pulsatingdirect current, the amplitude of which varies aecording to themodulation envelope. The pulsations in the signal are removed in thefilter comprising the input condenser 68, the choke 69 and followed bythe series choke and condenser '!0H connected between the conductor 12and ground. Thus at 13, there remains only a signal which correspondsexactly to the modulation envelope of the original signal.

The next step in the procedure is to use this signal energy toreconstruct an 1800 cycle amplitude modulated signal in order that itmay be supplied to a conventional facsimile receiver and there used toreproduce the picture.

For generating the 1800 cycle carrier signal, I,

provide an 1800 cycle audio oscillator 80 which may be of any standardtype giving a good sine wave and having a constant frequency output of1800 cycles. The output from the oscillator is connected in push-pull tothe grids 8! of the tube 82, which may be a twin triode. The anodevoltage for this tube is obtained from. the signal (at 13) after itleaves the filter, the signal being supplied to the anodes throughresonant filters, each of which comprises an inductance B4 and 85paralleled by capacities 85 and 8'! respectively. The two inductancesare connected end to end and, as stated, each has a condenser across it.The signal is fed to the junction of the inductances and capacities. Theleads 88 and 89 connect to the anodes of the tube 82. The two resonantfilters 84-85 and 8B81 are tuned to the frequency of the oscillator 80.Tube 82, which may be termed a restorer tube, only operates to pass asignal when plate voltage is supplied to the anodes. Because the platevoltages on this tube are being supplied by the signal which issubstantially a replica of the original signal used to modulate thecarrier at the facsimile transmitter, the signal in the plate circuit ofthe tube varies with the plate voltage. The signal as it appears acrossthe parallel resonant circuits is an 1800 cycle amplitude modulatedcarrier which is supplied through the coupling condensers 90 to the:potentiometers 9! which are connected in series and. grounded at theirjunction. The sliders 92 of the potentiometers are connected to thecontrol grids 93 of the twin triode tube 94 which operates in push-pull.The cathodes of the tube are grounded through the resistor 95. The platecircuit of the tube is connected to the output transformer 96. The tube94 amplifies the signal which is thus supplied to the transformer 96.The level of the signal may be determined by a meter M indicatordisposed in the output. The output from the in- 6 verter maybe connecteddirectly to a standard picture receiver It for reproduction or it may besent over land wires to the place where the picture receiver is located.

It should particularly be noted that the filter network ahead of thepoint 13 and comprising the inductance 69 and capacity 68, and theinductance 10 and capacity H are effective to prevent anything but theDC component of the desired signal from getting through to the point 13,and that the resonant circuits 84-85 and BIS-81 are resonant to 1800 andtherefore present a high impedance to the 1800 cycle carrier andtherefore readily transfer the 1800 frequency through the couplingcondensers to the. input circuit of the tube 93. ily admit the DC energyfrom 13 to the anodes of tube 82.

Thus the signal in the restorer circuit is a clean cut signal withoutany spurious modulationsdue to the undesirable elements in the incominsignal. r

The two taps shown for the output meter provides means whereby when agreater resistanc'ejis in series, the gain can be raised and the properlevel determined for sending the picture by land wire. If the picture isfed directly to the picture receiver, the gain ma be lowered and theproper level determined by shifting the: meter to the tap where thelesser resistance is in the circuit. In either case the meter enablesthe proper output tobe determined.

Having thus described my invention, I am aware that numerous andextensive departures may be made therefrom without departing from thespirit or scope of the invention.

I claim:

1. A limiter circuit comprising a signal input circuit having anelectron tube including a grid, a cathode and an anode, a couplingresistor coupling said circuit to the grid of said tube, a secondelectron tube including a cathode, grid and anode arranged to normallydraw current and said first tube being directly connected to the secondtube and the anode voltage for the first tube being supplied by thesecond tube; said input circuit of the first tube adapted, upon a signalbeing impressed thereon, to transfer said signal through said couplingresistance to the first tube and to drive said grid positive and saidresistance upon the grid being driven positive and drawing current tocause a voltage drop thereacross and effective to reduce the signal onthe grid, and said anode of the first tube converting the positivesignal to a negative signal to drive said grid of said second tubenegative to reduce the plate current in the second tube and hence reducethe plate voltage of the first tube to severely limit said signal.

2. A limiter circuit comprising a signal input circuit having anelectron tube including a grid, a cathode and an anode, a couplingresistor coupling said circuit to the grid of said tube, a secondelectron tube including a cathode, grid and anode arranged to normallydraw supply current and said first tube being coupled to the second tubeand the anode voltage for the first tube being supplied by the secondtube; said input circuit of the first tube adapted, upon a signal beingimpressed thereon, to transfer said signal through the said couplingresistor to the first tube to drive said grid positive, said resistance,upon the grid being driven positive, drawing grid current to cause avoltage drop thereacross and. effective to reduce the signal on theHowever, these resonant circuits readawa eoi 7 grid, said anode of thefirst tub converting the positive signal to a negative bias to bias saidgrid of said second tube negative to reduce the plate current in thesecond tube and hence reduce the plate voltage of the first tube toseverely limit said signals.

.3. A limiter circuit including a signal input circuit comprising anelectron tube having a grid, a cathode and an anode, a couplingresistor.disposed in the grid circuit of said tube, a second electrontube having a cathode, grid and anode arranged to normally draw current,said first tube being connected to the second tube, the anode voltagefor the first tube being supplied by the second tube; said input circuitof the first tube adapted upon a signal being impressed thereon to passsaid signal through th grid resistance of the first tube and to drivesaid grid positive andsaid resistance, upon grid being driven posi'-,tive, causing a voltage drop thereacross to reduce 4- .A-limitercircuit comprising a signal input circuit having an electron tubeincluding a grid, a cathode and an anode, a coupling resistor couplingsaid circuit to the grid of said tube, a second electron tube includinga grid, a cathode and anode arranged to normally draw supply current andsaid first tube being coupled to the second tube and the anode voltagefor the first tube being supplied by the second tube; said input circuitof the first tube adapted upon a signal being impressed thereon totransfer said si nal through said coupling resistor to the first tube todrive said grid positive and said resistor, upon the grid being drivenpositive and drawing said current, to cause a voltage drop thereacrossand efiective to reduce the signal on the grid, said anode of the firsttube converting the positive signal to a negative bias .to drive saidgrid of said second tube negative to reduce the plate current in thsecond tube and hence reduce the plate voltage of the first tube toseverely limit said signals.

' LOUIS Av THOMPSON.

No references cited.

